The present invention relates to cleaning metal and the like by liquid contact; and more particularly, to an improved process and apparatus for removing oxides, millscale and other impurities from continuous strip metal traveling through plural stages of pickling and rinsing with recirculated solutions.
Objectionable oxides, millscale and the like formed on or adhered to the surface of steel after milling or metal working are usually removed by immersion in an acidic solution or so-called pickling. In a continuous pickling process the steel in strip form travels longitudinally through a series of pickling and rinsing solutions at a regulated speed. The efficiency with which this is accomplished is dependent on several factors including the temperatures of the strip and the solutions, acidity of the solutions, and duration the strip is immersed. Prior art systems have been found inadequate for controlling all of these factors with the precision needed to produce steel strip with high surface quality while realizing effective chemical waste treatment, conservation of acid and water, and uninterrupted processing. For example, U.S. Pat. No. 2,166,583 to Critten discloses a process in which the strip is longitudinally transported through a series of pickling and rinse tanks. Hold-down rollers in the tanks keep the strip immersed while pinch rolls at each end of the series keep the strip taut as it passes through each tank. Spent acid in any tank may be drained by gravity to a dump tank within the series and replaced by fresh acid without interrupting operation of the other tanks. U.S. Pat. No. 2,697,050 to Barnes discloses a similar process except the pickling solution flows by gravity through the tanks in the same direction as the strip travels and the acid in the last tank is recirculated to the first tank. Fresh acid is added to the first tank when needed to raise the overall acidity. In some processes the spent acid is also purified and regenerated before it is recirculated. U.S. Pat. No. 3,445,284 to Robinson Jr., et al, for instance, removes any accumulated solids and distills the remaining acid to a higher concentration.
There is still a growing demand for pickling processes which are even more efficient and less detrimental to the environment. Some of the prior art systems require extensive waste chemical treatment to ensure there is no harmful discharge into the air, streams or ground. Still others consume large amounts of acid, employ costly regenerating systems for acid conservation, or require large quantities of water. Also, many critical and expensive components within these systems are often short lived due to their continuous exposure to the pickling solutions, and therefore require frequent replacement. More precise control of acid concentration, strip and pickling solution temperatures and immersion time are also required for improved product quality.